The invention relates to a method for controlling the operating point of a cathode ray tube, to which a video signal is supplied and which further has a control grid that applies a control voltage to an adjustment unit for adjusting the operating point. The invention further relates to a circuit arrangement for implementing this method.
German Patent DE 36 10 190 discloses a method for controlling the operating point of the video output stages of a monitor. To this end, a beam current is measured during selected lines that are not required for the display of information on a screen and is used to control the operating point. A voltage that is proportional to the beam current is measured at a resistor, which is connected to ground as well as to a cathode via a transistor. A video amplifier evaluates the voltage to control the operating point.
German Application 197 46 426.2 proposes means for controlling the operating point, which measure brightness values in at least a portion of an invisible area while a test signal is being injected. Said controlling means converts the measured brightness values into a controlled quantity, which can be supplied to a control unit to control the operating point.
An object of the present invention is to improve the operating point control in a method and circuit arrangement of the type mentioned above.
This and other objects of the invention are achieved, with respect to a method of the invention, by controlling the operating point of a cathode ray tube, to which a video signal is supplied and which includes a control grid that applies a control voltage from an adjusting unit for adjusting the operating point. Specifically, during a calibration phase using a test image for a set operating point with different brightness and/or contrast values the method includes supplying a test video signal to a cathode ray tube model and to a cathode ray tube; determining an emulated beam current from the video signal using the cathode ray tube model; measuring a beam current of the cathode ray tube, which is proportional to the video signal; calculating first characteristic quantities from the measured beam current and the emulated beam current; and storing said first characteristic quantities calculated from the measured and emulated beam currents in a memory location.
During normal operation, when the cathode ray tube is controlled with picture signals the method additionally includes supplying a picture video signal to the cathode ray tube model and to the cathode ray tube; determining an operational beam current from the picture video signal using the cathode ray tube model; measuring an operational beam current of the cathode ray tube, which is proportional to the picture video signal; calculating second characteristic quantities from the measured operational beam current and the operational emulated beam current; comparing the second characteristic quantities with the first characteristic quantities stored in the memory; and adjusting the control voltage until the second characteristic quantities substantially match the first characteristic quantities stored in the memory during the calibration phase.
The objects are also achieved by a circuit arrangement for controlling the operating point of a cathode ray tube, to which a video signal is supplied and which includes a control grid that applies a control voltage to the cathode ray tube. Specifically, the circuit arrangement includes a cathode ray tube model (a) that determines an emulated beam current from a test video signal during a calibration phase using a test image for a set operating point with different brightness and contrast values and that (b) determines an operational emulated beam current from a supplied video signal during normal operation; an arithmetic unit that (a) calculates first characteristic quantities during the calibration phase from a measured beam current value and the emulated beam current, which is proportional to the video signal, and that (b) calculates second characteristic quantities during the normal operation from an operating measured beam current and an operating emulated beam current, which is proportional to the video signal; a memory for storing the first characteristic quantities; a comparator for comparing the second characteristic quantities determined during the normal operation with the first characteristic quantities stored in the memory during the calibration operation; and an adjusting unit that adjusts the control voltage until the second characteristic quantities substantially match the first characteristic quantities stored in the memory.
One important idea underlying the invention is to provide a model for controlling the operating point of a cathode ray tube. In other words, the cathode ray tube model emulates the behavior of the actual cathode ray tube. A video signal is supplied to both the model and the cathode ray tube. During a calibration phase, when a monitor is energized, characteristic quantities, e.g., in the form of quotients, are first determined from the real and the emulated beam currents with a previously set operating point using a given test image for various brightness and/or contrast values. These characteristic quantities are stored in a memory of an adjustment unit. Characteristic quantities are also determined from the real and the emulated beam currents during normal operation of the monitor, i.e., when the cathode ray tube is controlled by corresponding picture signals. These characteristic quantities are compared with the characteristic quantities stored in the memory. If the respective characteristic quantities differ from one another, which is mostly attributable to aging effects of the cathode ray tube, the characteristic quantities for each image to be displayed are adapted to the ones stored in the memory by adjusting the control voltage accordingly.
Advantageously, the invention makes it possible to control the operating point of the cathode ray tube while an image is being displayed, i.e., while the cathode ray tube is controlled by a corresponding picture signal. Injection of a test signal, which would disturb a viewer, is not required. In addition, the measurement as the basis for control is carried out without negatively affecting the frequency response of the video amplifier, which has a positive effect on the bandwidth of the monitor.